1. Field of the Invention
The present invention relates to an optical modulator, and more particularly, to a variable chirp modulator having a three-arm interferometer. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for controlling a chirp in operating an optical modulator.
2. Discussion of the Related Art
An integrated optical modulator is of great interest in operating a fiber optical communication system in the range of 2.5 to 10 Gbps (Giga bits per second), and potentially in the range of 40 Gbps or above. A great deal of research has been carried out to quantify signal frequency broadening for different types of modulation. This effect is known as chirp. Chirp can cause loss of signal fidelity after propagating down the optical fiber due to a wavelength dispersion. In other words, chirp interacts with the dispersion profile of the transmission fiber to severely limit the distance over which error-free data is possible.
There has been an effort to provide either fixed (zero or non-zero) or variable chirp by varying an arrangement of the system. Generation of chirp is system dependent, and it may include zero chirp, variable chirp, or chirp at a fixed non-zero value. Generally, modulators with variable chirp are more complicated, or require a more complex electrical driving scheme than fixed chirp modulators.
A conventional approach to control a chirp in external modulators has been to use an amplitude modulator and a phase modulator in series. They are driven with adjustable voltages or pre-selected electrode lengths are used to obtain a desired value of chirp. A disadvantage of this approach is that the series configuration of the modulators generally leads to higher drive voltages due to a limited available device length. Fixed, non-zero amounts of chirp can be obtained in a single Mach Zehnder amplitude modulator in which the arms of the interferometer are driven with independent drive voltages with adjustable amplitude and phase. A drive voltage or a voltage required to operate the modulator from an off-state to an on-state is one of the important features for external modulators. By minimizing the voltage, drive power required to operate the modulator can be minimized.
For Mach Zehnder amplitude modulators biased at their quadrature or linearly operating point, zero chirp can be obtained by driving the arms of the interferometer in a symmetrical fashion, so that the light in each arm receives equal and opposite phase shifts. One way to achieve this feature is to apply equal and oppositely directed electric fields to each arm of the interferometer, while ensuring that the electro-optic overlap integrals are the same for each arm. Fixed non-zero chirp may be created by varying a magnitude of the field, a magnitude of the overlap integral, or both in one of the interferometer arms. The conventional art does not allow for a device in which zero and adjustable values of non-zero chirp can be achieved in a single device.
Accordingly, the present invention is directed to a variable chirp modulator having a three arm interferometer that substantially obviates one or more problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide an optical modulator controlling a chirp in operating an optical communication system.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an electrically tunable optical modulator includes a substrate having an electrooptical effect, an optical waveguide having first, second, and third cascading portions in the substrate, and transmitting an optical field, a first coplanar waveguide electrode having a first part over the first cascading portion and second and third parts extending beyond the first cascading portion, a second coplanar waveguide electrode having a fourth part over the second cascading portion and fifth and sixth parts extending beyond the second cascading portion, a third coplanar waveguide electrode having a seventh part over the third cascading portion and eighth and ninth parts extending beyond the third cascading portion, a fourth coplanar waveguide electrode formed to have a first distance from the first coplanar waveguide electrode, a fifth coplanar waveguide electrode formed between the first and second coplanar waveguide electrodes to have second and third distances, respectively, a sixth coplanar waveguide electrode formed between the second and third coplanar waveguide electrodes to have fourth and fifth distances, respectively, a seventh coplanar waveguide electrode formed to have a sixth distance from the third coplanar waveguide electrode, and a voltage source supplying voltages to the first, second, and third coplanar waveguide electrodes, wherein the fourth to seventh coplanar waveguide electrodes are grounded and the first to sixth distances are substantially the same, so that electrooptic overlap integrals of each cascading portion of the optical waveguide are the same.
In another aspect of the present invention, a wavelength division multiplexed optical transmission system having an electrically tunable optical modulator, the modulator includes a substrate having an electrooptical effect, an optical waveguide having first, second, and third cascading portions in the substrate, and transmitting an optical field, a first coplanar waveguide electrode having a first part over the first cascading portion and second and third parts extending beyond the first cascading portion, a second coplanar waveguide electrode having a fourth part over the second cascading portion and fifth and sixth parts extending beyond the second cascading portion, a third coplanar waveguide electrode having a seventh part over the third cascading portion and eighth and ninth parts extending beyond the third cascading portion, a fourth coplanar waveguide electrode formed to have a first distance from the first coplanar waveguide electrode, a fifth coplanar waveguide electrode formed between the first and second coplanar waveguide electrodes to have second and third distances, respectively, a sixth coplanar waveguide electrode formed between the second and third coplanar waveguide electrodes to have fourth and fifth distances, respectively, a seventh coplanar waveguide electrode formed to have a sixth distance from the third coplanar waveguide electrode, and a voltage source supplying voltages to the first, second, and third coplanar waveguide electrodes, wherein the fourth to seventh coplanar waveguide electrodes are grounded and the first to sixth distances are substantially the same, so that electrooptic overlap integrals of each cascading portion of the optical waveguide are the same.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.